CLEVELAND — A tape recording of the shooting deaths of four Kent State University students by Ohio National Guardsmen in 1970 reveals the sound of pistol shots 70 seconds earlier, a newspaper reported Friday, citing the work of a forensic audio expert.

If the pistol fire is authenticated, it could prove a theory that the Guardsmen thought they were being shot at during a campus Vietnam War protest and also could back up witnesses who said an FBI informant monitoring the protest fired warning shots because he felt threatened.

The National Guard opened fire on student protesters on May 4, 1970, killing four and injuring nine others. Eight Guardsmen were acquitted of federal civil rights charges four years later.

Many believe the events contributed to the change in the public’s attitude toward the war, which ended with U.S. withdrawal in 1975, but the events of that chaotic day in Kent, Ohio, are still not fully understood.

Forensic audio expert Stuart Allen has conducted an extensive review of the tape recording and detected four shots matching the acoustic signature of a .38-caliber revolver firing, The Plain Dealer reported.

Alan Canfora, a protester wounded by the Guard gunfire, found a copy of the audio tape in a library archive in 2007.

“I think it’s premature to make any conclusions at this point,” said Canfora, who nevertheless believes questions posed by the analysis add pressure on government officials to open a new investigation.

Terry Norman, a Kent State student who was photographing protesters that day for the FBI, was carrying a loaded .38-caliber revolver under his coat, the newspaper said.

Witnesses have previously reported a confrontation involving angry students and Norman. Some say he fired several warning shots because he felt threatened.

In an interview with an Akron Beacon Journal reporter on the day of the shootings, Norman said he was carrying the pistol for protection because protesters threatened his life when he photographed earlier sit-ins. He has denied firing it, and the presidential commission that investigated the shootings determined that Norman played no role in them.

A crew from Cleveland’s WKYC-TV filmed Norman running toward Guardsmen and police the day of the shooting and being chased by two men. One of the men yelled: “Hey, stop that man! I saw him shoot someone!”

The crew recorded Norman reaching under his jacket and handing a gun to a police officer, saying “the guy tried to kill me.” Norman later repeatedly said he was referring to an assault that happened after the Guard shootings.

Former WKYC television reporter Fred DeBrine and sound man Joe Butano have said repeatedly that they heard a Kent State police detective open the cylinder of Norman’s gun and say: “Oh my god, he fired four times.”

The police detective later denied making the remark, and a campus patrolman’s report said the gun was fully loaded with no smell of burnt powder.

Debrine and Joe Butano repeated their assertion this week, The Plain Dealer reported. Butano also has said he heard the four shots and the Guard’s rifle volley soon afterward. The pistol fire on the audio would verify “what I heard and have been thinking about all these years,” Butano told the newspaper.

The newspaper said Norman has remained elusive for decades. He could not be reached for comment.

The reel-to-reel audio recording was made by a student who placed a microphone at a window sill of his dormitory that overlooked the anti-war rally. He later turned the tape over to the FBI, which kept a copy that wound up in a Yale University archive.

Allen, the audio expert, and a colleague first examined the recording earlier this year at the request of the newspaper. They earlier concluded that someone ordered the troops to prepare to fire on students, but they were unable to determine who issued the command or why.